Key Idea: Topic 10.1 is the geophysical systems that produce the hazards in Option D — how the Earth's restless crust generates earthquakes, volcanoes and mass movements. It pulls together two ideas: 10.1.1 — plate tectonics, earthquakes & volcanoes: the Earth's plates meet at margins, and the type of margin (constructive, destructive, conservative) controls where these hazards happen and what they are like — runny vs explosive lava, deep vs shallow quakes, and the tsunami sequence. 10.1.2 — mass movement: the downhill movement of rock, soil and debris under gravity, classified by speed and water content, and tipped into failure by physical factors and a trigger — often as a secondary hazard. This is an Option D topic, examined on Paper 1. SL answers 2 options, HL answers 3 (same questions). Expect short data-response reads off a hazard map/graph, a cluster of structured parts (Outline, Identify, Describe), and a [10] Examine essay on plate margins and hazard character.
🌋 10.1.1 — Plate tectonics, earthquakes & volcanoes
The Earth's rigid shell is broken into tectonic plates that drift on the asthenosphere, driven by mantle convection. Almost all earthquakes and volcanoes cluster along plate margins — so the margin type sets the character of the hazard. The skill examiners test is linking a margin type to a hazard's character (lava type, explosiveness, landform; or earthquake depth and magnitude), plus the tsunami sequence and why the largest quakes are rare.
The three plate margins and what each produces
| Margin type | Plate movement | Volcanoes | Earthquakes |
|---|---|---|---|
| Constructive (divergent) | Move apart; magma rises to fill the gap | Frequent, gentle (effusive); runny basaltic lava builds shield volcanoes / ridges | Frequent but small and shallow |
| Destructive (convergent) | Move together; oceanic plate subducts | Explosive; sticky andesitic lava builds steep composite cones | Large, can be deep; cause the biggest quakes (and tsunamis) |
| Conservative (transform) | Slide past one another | None — no magma is created | Large but shallow; sudden release of built-up strain |
Constructive margins melt the mantle directly → low-silica, runny basaltic lava → gas escapes → gentle eruptions that spread far (shield volcanoes). Destructive margins melt subducted crust → high-silica, sticky andesitic lava → gas is trapped → explosive eruptions that build steep composite cones. Hotspots (e.g. Hawaii) and intraplate quakes occur away from margins, so plate movement is a strong control — not an absolute one.
Key terms — tectonics & hazards
- Tectonic plate — a rigid slab of crust and upper mantle that moves over the asthenosphere.
- Plate margin (boundary) — where two plates meet; almost all earthquakes and volcanoes occur here.
- Constructive / destructive / conservative — plates move apart / together (subduction) / past each other.
- Focus / epicentre — where an earthquake starts underground / the point directly above it on the surface.
- Tsunami — radiating sea waves formed when an undersea event suddenly displaces the sea floor.
- Hotspot — a mantle plume producing volcanoes away from any margin (e.g. Hawaii).
A typical Paper 1 figure: read the key first. Risk is highest at the vent and the lahar valley and falls outward — quote a figure with its units.
🔒 Interactive diagram
Explore the labelled diagram, charts and maps for this topic in study mode.
⛰️ 10.1.2 — Mass movement processes
Mass movement is the downhill movement of rock, soil and debris under gravity — a slope fails when the driving force (gravity) beats the resisting force (friction and material strength). Types are classified by speed and water content, from imperceptible soil creep to a deadly debris flow or rock fall. The examiner skill is reading a classification graph (slowest type = soil creep; a slope-steepness value off the axis) and giving a mechanism in every Outline — a factor plus how it works.
Mass movement types — by speed and water content
| Type | Speed | Water content | What it looks like |
|---|---|---|---|
| Soil creep | Very slow (mm/yr) | Low | Soil edges downhill — tilted poles, curved tree trunks, terracettes |
| Slump | Slow to moderate | High | A block rotates along a curved slip surface, leaving a back scar |
| Slide | Fast | Moderate | Rock or soil slides as a block down a flat slip plane |
| Flow (mud/debris) | Fast to very fast | Very high | Saturated soil and rock flow like a fluid downslope |
| Rock fall | Very fast | Low (often dry) | Blocks detach from a cliff and fall or bounce to the base |
What tips a slope into failure
| Physical factors (set instability) | Human activities (destabilise) | |
|---|---|---|
| What they do | Add weight/water or remove strength on the slope itself | Remove support, add load or add water through human action |
| Examples | Water content, steep gradient, loose/weak sediment, geology, bare vegetation | Undercutting the base, building on slopes, deforestation, waste heaps, disturbing drainage |
| The mechanism | Water adds weight + lubricates the slip surface → faster movement | Undercutting removes support against gravity → the slope above fails |
Key terms — mass movement
- Mass movement — the downslope movement of rock, soil and debris under gravity.
- Slope stability — how resistant a slope is to failing; high friction and strong material = stable.
- Slip surface — the curved or flat plane along which material breaks away and moves.
- Speed of onset — how fast a hazard strikes; creep is slow, a flow or fall is almost instant.
- Trigger — the final factor (heavy rain, an earthquake, undercutting) that tips a slope into failure.
- Primary vs secondary hazard — the original event vs one it triggers (e.g. a quake that sets off a rock fall).
Important: A rock fall or landslide is a secondary hazard when another event triggers it — an earthquake or heavy rain dislodging loose blocks. The quake is the primary hazard; the fall it sets off is the secondary one. This links the two micros: the same earthquakes and eruptions of 10.1.1 are the triggers that set off the mass movements of 10.1.2 (e.g. the Nevado del Ruiz lahar, 1985).
✍️ IB-style questions
Explain how the type of plate margin influences the character of the volcanoes that form there.
🔒 Model answer plan
See the mark-by-mark plan — for / against / judgement, with marking guidance — in study mode.
Examine how the type of movement at different plate margins controls the characteristics and distribution of geophysical hazards.
🔒 Model answer plan
See the mark-by-mark plan — for / against / judgement, with marking guidance — in study mode.
✅ Quick self-check
Tap each card to reveal the answer.
Why are destructive-margin volcanoes so explosive? Subduction makes sticky, high-silica andesitic magma that traps gas, so it bursts out violently and builds steep composite cones — unlike the runny basaltic lava of a constructive margin.
How does a tsunami form? An undersea earthquake, eruption or landslide suddenly displaces the sea floor, pushing a huge volume of water; waves radiate out and grow tall as they reach shallow coasts.
Which mass movement moves most slowly, and how do you spot it? Soil creep — millimetres a year. You spot it from tilted telegraph poles, curved tree trunks and terracettes, not a sudden scar.
Why does water content speed up a mass movement? Water adds weight and lubricates the slip surface, reducing friction between grains — so saturated material moves faster and more fluidly.
When is a rock fall a secondary hazard? When it is triggered by another event — an earthquake or heavy rain dislodges loose cliff blocks. The quake is primary; the fall it sets off is secondary.
🎯 Highest-yield exam reminders
Exam Tips
- Margin type controls the hazard: destructive = violent (explosive cones + biggest quakes/tsunamis), constructive = gentle (shields + small quakes), conservative = quakes only.
- Lava follows magma: low-silica basaltic = runny shield that spreads far; high-silica andesitic = explosive steep cone.
- Tsunami = sudden sea-floor displacement → waves radiate out → grow at the coast. Large quakes are rare because strain takes a long time to build.
- Mass movement = rock/soil/debris downhill under gravity; slowest = soil creep, fastest/wettest = debris flow / rock fall.
- Outline [2] is never just a name: factor → how it works → effect on the slope. Half the marks are the mechanism.
- On the [10] Examine: contrast two or more margins linked to hazard character + a named example + the hotspot/intraplate exception + a clear judgement.